1. Field of the Invention
The subject invention relates to a converter used in a system for monitoring the condition of a vehicle wheel mounted tire such as the pressure or temperature within the tire.
2. Description of the Prior Art
For some time there has been interest in monitoring the air pressure in a pneumatic tire. Because an important factor governing the useful service life of a pneumatic tire is correct inflation pressure, there has been significant interest with truck tires because of their relatively high replacement cost. There is now, however, significant interest in monitoring the tire inflation pressure on passenger cars because correct inflation pressure significantly improves fuel economy by decreasing tire rolling resistance.
Many systems have been proposed to continuously monitor tire pressure on a vehicle but none have been widely adopted because of difficulties with either cost, complexity, reliability or a combination of these factors.
The prior systems have either been mechanical or electrical with the mechanical systems having been virtually eliminated from consideration. Electrical systems have included a battery powered wheel monitor radio transmitter or a passive circuit of the energy absorbing type or an induction or transformer type. The battery system is such that the system draws power from the battery only when a fall in pressure or temperature occurs to power the transmitter. These systems have not been widely accepted because they are not fail-safe in that if a malfunction occurs in the system there is no indication. In a fail-safe system, the monitor must be active and self-checking until a fault is sensed, such as the drop of the tire pressure below a predetermined pressure. If a battery were utilized in such a system, it would have to be continually recharged because the system would be providing a signal at all times except when a fault is sensed.
Passive circuits have been used to monitor a tire condition wherein a passive converter circuit is responsive to an energy field, such as a radio frequency or magnetic field, and converts or transponds that signal to an energy field which may be detected or received. Passive circuits which have been utilized to sense tire pressure have not had the desired sensitivity and have inherent coupling problems. Furthermore, coupling to the detector or antenna in such systems is a function of the rotational position of the wheel. The subject invention relates to a passive circuit converter to be disposed within the tire and which overcomes many of the problems associated with the prior art systems.